An electric power steering controller disclosed in a patent document, Japanese Patent Laid-Open No. JP 2013-52793 A (patent document 1) estimates a load of a steering mechanism from a steer torque and from an assist torque instruction value, and sets a target steer torque based on the estimated load. Then, for the matching of the steer torque with the target steer torque, the controller calculates an assist torque instruction value based on a deviation of the steer torque from the target steer torque.
It is generally understood in the field of steering assist mechanism that, when a friction among the components in the steering mechanism or steering system components (which are a control object of the electric power steering controller) increases, a steering torque for steering a steering wheel also increases. Therefore, based on a technique used in the patent document 1, the assist torque instruction value must be increased with an increase of the friction between the components in the steering mechanism. Further, an increase of the assist torque instruction value that is used for the estimation of the load of the steering mechanism leads to an increase of the estimated load.
Under the influence of the friction, since the target steer torque in a turning state is increased, the steer torque increases to have a large value.
In a steering wheel return situation where the steering wheel is steered back to a neutral position, under the influence of the friction, steering torque decreases to zero before the steering wheel angle returns to the neutral position.
In other words, a returnability of the steering wheel (i.e., steering angle returning to 0 degree) becomes poor under the influence of the friction. That is, the driver has to apply additional force to the steering wheel to bring the steering wheel back to the neutral position.
The influence of the friction on the steering system components is particularly noticeable on a road with low coefficient of friction (μ) (low μ road), since a road surface reaction force is small. In other words, the influence of friction on a low μ road is large compared with a high μ road.
In other words, when steering the steering wheel on a low μ road (e.g., in a turning state or a returning state), the increase of the steer torque is devastatingly large in comparison to a low friction time/case. This also means that the returnability of the steering wheel back to neutral position becomes poor (i.e., is worsened) during a high friction time in comparison to the low friction time.
Further, in a low-speed travel time of the vehicle, a self-aligning torque is small as compared with a high-speed travel time, which also leads to an increase of the friction influence in the steering system components in the low-speed travel time as compared with the high-speed travel time. Therefore, as compared with the high-speed travel time, the steer torque for steering the steering wheel increases in the low-speed travel time, and the returnability of the steering angle worsens.
In the conventional steering system, when the influence of the friction of the steering system components changes due to the change of the friction itself or due to the other factors such as the change of the road surface μ and/or the vehicle speed, a steering feel changes. That is, when the influence of the friction changes relative to the deviation of the friction itself or the deviation of the other factors, the steering feel of the steering wheel changes in the conventional system.